The present invention relates to illumination apparatuses for using light from a light source to illuminate an illuminated plane, and more particularly to an illumination apparatus for illuminating a reticle or a mask (these terms are used interchangeably in the present application) which forms a pattern, in an exposure apparatus used in a photo-lithography process for fabricating semiconductor devices, liquid crystal display devices, image pick-up devices (CCD, and the like), thin-film magnetic heads, and the like.
In recent years, a demand on minute semiconductors has been increasingly stronger, and the minimum critical dimension has become less than 0.15 μm, approaching to 0.10 μm. The fine processing needs uniform illuminance to illuminate the mask, and a uniform effective light source distribution as an angular distribution of the exposure light to illuminate the mask and wafer, as well as a shorter wavelength of exposure light and an increased NA in a projection lens.
For uniform illumination of the mask without the uneven illuminance and the uniform effective light source distribution, a conventional optical system has used an illumination apparatus including two or more fly-eye lenses (including a combination of a glass rod lens or a cylindrical lens) or an internal reflection member and a fly-eye. In these configurations, the fly-eye lens of a back stage illuminates the mask plane evenly for the uniform illuminance, and the former fly-eye lens or the internal reflection member of the front stage illuminates the fly-eye lens of the back stage evenly for the uniform effective light source.
Uniform light that has a light intensity distribution with sharp edges enters the light incidence plane of the fly-eye lens of the back stage in this optical system. However, the uneven illuminance occurs when the edge of the light intensity distribution of the incident light enters only part of a rod lens in the fly-eye lens. A description will now be given of this problem with reference to FIGS. 14 and 15. Here, FIG. 14 is a schematic plan view showing a relationship between a light incidence plane of the fly-eye lens and light incident upon it. FIG. 15 is a sectional view taken along the longitudinal axis in FIG. 14 and illustrates a relationship among the fly-eye lens of the subsequent stage, the light intensity distribution of the incident light, and a light intensity distribution on the illuminated plane.
As shown in FIGS. 14 and 15, a fly-eye lens 20 includes five rod lenses 26a–26e defined by five wide lines (which are generalized by reference numeral 26 hereinafter unless otherwise specified), and receives incident light 10. FIG. 14 shows that the rod lens 26 has a square section vertical to the optical axis but, as described later, it may have a hexagon and other sectional shape. The light intensity distribution 12 also has a square shape for simplicity.
In forming a secondary light source at a light exit plane 24 of the fly-eye lens 20 and using the light from the secondary light source to Koehler-illuminates the target plane 40 through a condenser lens 30, a light incidence plane 22 of the fly-eye lens 20 is made optically conjugate with the illuminated plane 40 (i.e., in a relationship of an object plane and an image plane). Therefore, the light intensity distribution on the illuminated plane 40 is created by superimposing the light intensity distribution on the light incidence plane 22 of each rod lens 26 onto the illuminate plane 40.
For convenience, incident light 10 has a light intensity distributions 12a–12e (hereinafter, unless otherwise described, the reference numeral 12 generalizes them) corresponding to the rod lenses, but it will be understood that the incident light 10 corresponding to the light intensity distributions 12a and 12e enters only part of the rod lenses 26a and 26e. 
Thus, when the edge of the light intensity distribution 12 of the incident light 10 entering the fly-eye lens 20 crosses the rod lens, the illuminated plane 40 comes to form the light intensity distribution 50 that includes five superimposed light intensity distributions 52a–52e as illustrated at the right side of the illuminated plane 40. Since the light intensity distributions 12a and 12e incident upon the rod lenses 26a and 26e at both ends are cut on the way, the illuminated plane 40 has the light intensity distributions 52a and 52e, and the synthesized light intensity distribution 50 has an uneven light intensity distribution lacking the part 53. As is understood from the configuration of the fly-eye lens shown in FIG. 14, the rod lenses 26 line up in a direction perpendicular to the paper of FIG. 15, and thus the stack of all of these rod lenses 26 will further intensify the concave part 53 of the light intensity distribution 50.
As an actual example of the light intensity distribution 50, FIGS. 17 and 18 show a result of the light intensity on the illuminated plane 40 when the edge of the light intensity distribution 10 entering the fly-eye lens 20 traverses the rod lens 26. Here, FIG. 17 shows a light intensity distribution on the illuminated plane 40 when the rod lens 26 has a square section corresponding to a square illuminated area of the illuminated plane 40. FIG. 18 shows a light intensity distribution on the illuminated plane when the rod lens 26 has a hexagonal or circular section corresponding to a hexagonal or circular illuminated area of the illuminated plane 40. FIGS. 17 and 18 indicate, respectively, that darker part has a lower light intensity, and each of them shows that uneven light intensity distribution is produced due to the edge of an incident light intensity distribution. Color versions of FIGS. 17 and 18 will be attached to this application for better understanding of the invention.
As one solution for this problem, as shown in FIGS. 19A and 19B, it is conceivable to arrange a stop 60a or 60b near the light incidence plane 22 of the fly-eye lens 20 to shield the light intensity distributions 12a and 12e. Here, FIGS. 19A and 19B are plan views of the stops 60a and 60b, respectively. The stop 60a has a hollow rectangular shape defined by an outside outline 62a and inside outline 64a, and the stop 60b has a hollow shape defined by an outside outline 62b and inside outline 64b. The incident light 10's light intensity distribution 12 has a square shape, and its outline is blurred by the stops 60a and 60b as illustrated. As a result, the edge of the light distribution does not traverse the rod lens 26, but resides at the border of the rod lens 26.
However, a method for enhancing evenness of a light intensity at a conventional illuminated plane has lowered illuminance and throughput at a mask and a wafer as illuminated planes, which will, in the long run, increase product cost.
In other words, a stop used at a light incidence plane of the fly-eye lens for increasing an even light intensity distribution on an illuminated plane causes a loss of a light amount due to shield of the light by the stop.